Although the Bar instance reference is not retained by the Foo instance, the Bar instance is still technically a dependent instance of Foo and therefore the container needs to keep a reference to it in order to eventually destroy it properly.

This is a problem because dependent instances of a bean are currently supposed to be serialized together with the normal-scoped instance but the Bar instance is not serializable.

We may work around by making use of the following part of the spec:

Finally, the container is permitted to destroy any @Dependent scoped contextual instance at any time if the instance is no
longer referenced by the application (excluding weak, soft and phantom references)."

and destroy every non-serializable dependent bean instance of a normal-scoped bean when the creational context for the normal-scoped bean instance is serialized.

This is not 100% equivalent to "the instance is no longer referenced by the application" however covers all the sensible cases including:

Foo no longer holds a reference to Bar, therefore we can safely destroy Bar

Foo holds a reference to Bar - does not matter if we destroy Bar prematurely or not since serialization of Bar is going to fail anyway

Foo holds a reference to Bar in a transient field and is therefore either going to "forget" the reference no serialization (therefore we can destroy it) or is able to recreate the Bar instance itself on deserialization